Poly-2-pyrrolidone (also known as nylon-4) is composed of repeating C.sub.4 structural units --NH(CH.sub.2).sub.3 C(O)--. The solid, melt-spinnable homopolymer of 2-pyrrolidone is produced by the alkaline-catalyzed polymerization of 2-pyrrolidone in the presence of carbon dioxide, using a catalyst composed of alkali metal lactamate or tetraalkyl ammonium lactamate (see U.S. Pat. No. 3,721,652). Copolymers of 2-pyrrolidone with C.sub.5 -C.sub.6 lactams have C.sub.5 -C.sub.6 structural units substituted for some of the C.sub.4 structural units of poly-2-pyrrolidone. Copolymers of 2-pyrrolidone with caprolactam reportedly have been made by alkaline-catalyzed polymerization, see J. Polymer Science Part A, 1, 111 (1963) and Part A-1 5, 965 (1967), and Czech Patent 158,341.
It is an object of the present invention to achieve the rapid alkaline-catalyzed copolymerization of 2-pyrrolidone with C.sub.5 -C.sub.6 lactams in the presence of carbon dioxide. It is a further object of the present invention to produce a solid, melt-spinnable copolymer of 2-pyrrolidone and C.sub.5 -C.sub.6 lactams which contains more than about 2 mol percent and preferably more than 10 mol percent of C.sub.5 -C.sub.6 structural units. It is a further object of the present invention to produce a solid copolymer of 2-pyrrolidone and a C.sub.5 -C.sub.6 lactam possessing greater thermal stability than the solid 2-pyrrolidone homopolymer.